Programmed robots, master-slave manipulators, telemanipulators, and material handling devices, in general, have a need to measure torque in articulated joints so that force and position control may be precisely exercised. For gripping devices, maximum gripping forces may be varied according to the nature of the object being grasped. In assembly operations involving a wise range of variables, adaptive control is dependent upon force and position feedback information. Where unplanned contact with an object is a risk, force feedback can be used to initiate corrective action and to prevent damage to the manipulator and/or to the object being grasped.
These manipulator devices also have size constraints as minimum weight and bulk become increasingly important in the area of the end effector. Where there are size constraints, various indirect means are now used to measure torque in a joint. These include: displacement measurement of a resilient element in the drive mechanism; measurement of actuator motor currents or other actuator parameters; force sensors in drive mechanisms; and strain gages in structural members.
One example of a resilient element in the drive mechanism is shown in U.S. Pat. No. 4,600,357 to Coules. This patent discloses a robot end effector with two opposing pivoted jaws. A helical gear assembly connects these jaws to a single rotary drive motor with a resilient element incorporated into the drive shaft. Rotary position sensors are incorporated into the drive shaft before and after the resilient element. The difference between the rotary position sensors provides an indication of the torque being transmitted.
U.S. Pat. No. 4,727,996 to Fenn et al. discloses a gripping mechanism where the actuator motor current is sensed as an indirect indication of the gripping forces. Only a dynamic reading is provided. As changing ambient conditions can effect the torque force, critical information is not available when the motor is inactive.
U.S. Pat. No. 4,666,361 to Kitabatake et al. discloses the use of force sensors to indirectly measure torque. A rotary motor is used to drive a worm wheel assembly which is connected to an arm joint by means of a drive belt. Force sensor assemblies are located at opposite ends of the worm gear. An indication of the torque at the arm joint is provided by measuring the axial force present in the worm gear. The direction of the torque in the arm pivot is indicated by which force sensor is active. The force sensors can continue to monitor the torque while the motor is inactive.
Strain gages for indirect torque measurement are used in advanced hand-like grasping devices with multiple and closely spaced joints. An example is the Stanford/JPL hand-like robot end effector. (International Encyclopedia of Robots. R. Dorf, Editor. 1988, page 627). Drive motor actuating forces are transmitted by cables in flexible conduits from the drive motors located beyond the end effector wrist. An indication of torque at each joint is provided by strain gages incorporated in the cable guiding structure.
In all these preceding examples of indirect measurement of torque, errors can be introduced by friction and tolerance variations in the mechanical linkage between the sensor and the joint. Also, the effects of gravity and inertia have to be recognized as the actual torque present may be but one component of the indirect sensor measurement.
As operating speeds increase, the adverse impact of inertia becomes of increasing importance in indirect measurement of torque. (J. K. Salisbury et al, "Determination of Manipulator Contact Information from Joint Torque Measurements", Experimental Robotics I, 1990).
To measure torque directly at the joint, Stanford University's Robotics Laboratory has placed strain gages on the spokes of a final drive wheel which is concentric with the joint axis. The Laboratory, as an alternative, also has placed a complex of four contact-free distance sensors to measure the beam deflection of the spokes of the final drive wheel. ("Design and Development of Torque-Controlled Joints", Experimental Robotics I, 1990, pages 281-283). The need to provided a drive wheel with spokes sufficient in length for the placement of strain gages or contact-free distance sensors limits the minimum overall size of the drive wheel. In the reference cited, the drive wheel was added externally to the joint and increased the overall size of the joint.
T. Oomichi et al ("Mechanics and Multiple Sensory Bilateral Control of a Fingered Manipulator", Robotics Research 1988, page 149) describes force sensors embedded between the finger joints. Forces at right angle to the center-line of the fingers can be measured in two directions and thus provide a reading of the torque present and its direction. The embedded sensor becomes in effect a structural member of the finger and reduces the space available for the routing of actuator mechanisms.
In my pending patent application "Manipulator Integral Force Sensor", Patent and Trademark Office Ser. No. 07/614,099, a displacement sensor and a resilient element are integral with the shaft of a joint for the direct measurement of torque.
The present invention provides a simple apparatus to measure torque directly at a joint and eliminate the errors introduced when indirect measurements are made of joint torque. The simplicity of the apparatus provides for a compact joint assembly essential for increasingly complex end effectors.
The present invention provides an alternative to that described in my pending patent application cited above.